ME 3101 Material Science Fall 2016

1. Main Topics this chapter...
• What promotes bonding?
• What types of bonds are there?
What
properties are
inferred from
bonding?
ME3101-Materials Science and Engineering
Ch2: BONDING AND PROPERTIES
Nucleus: Z = # protons- responsible for primary
classification of the material
orbital electrons:
n = principal
quantum number
n=3 2 1
= 1 for hydrogen to 94 for plutonium
N = # neutrons
Atomic mass A ≈ mass of nucleus = Z + N
Adapted from Fig.
2.1, Callister 6e.
BOHR ATOMIC MODEL
Negatively
charged electrons
spin around
positively charged
nucleus in
specified orbits.
WAVE MECHANICAL MODEL
• Every Electron- characterized by 4 quantum numbers (ZIP CODES !)
• They affect shape, size and orientation of electron’s probability density
in the orbit- where and when electrons may be found/exist
• Bohr levels – divide into number of electron shells- depends on
quantum number, which are:
1. Principal quantum number-1, 2, 3,.. Main energy states. ~ Bohr
shell # - designated K L M N
2. Subsidiary (Angular Momentum) Subshells within the main
energy states Standard designations… s, p, d, f
(sharp, principal, diffuse, and fundamental. Original appearance of spectral
lines)
3. Magnetic quantum number- number of energy states within
subshells (energy levels) in s:1 in p:3 in d:5 in f:7
4. Spin moment (similar to Clockwise/counterclockwise)
designated as + ½ - ½ Possible in every energy state
ELECTRONS IN SHELLS/SUBSHELLS
summary

2. • have discrete energy states
• tend to occupy lowest available energy state.
• notice overlap: 3d higher than 4s !
Increasingenergy
n=1
n=2
n=3
n=4
1s
2s
3s
2p
3p
4s
4p
3d
Electrons...
(in practice)
ELECTRON (relative) ENERGY STATES
SeeFiguresintext
• have complete s and p subshells
• tend to be inert/non-reactive.
• known as noble gas/elements.
Elements with stable electron configurations...
Z Element Configuration
2 He 1s2
10 Ne 1s22s22p6
18 Ar 1s22s22p63s23p6
36 Kr 1s22s22p63s23p63d104s24p6
Adapted from Table 2.2,
Callister
STABLE ELECTRON CONFIGURATIONS
Other elements will have different outer sub-shell configurations…
Review
1. How many quantum numbers? 1 2 3 4 6 8 12
2. What are they called? Principal, sub…, mag.., sp
3. What are the various levels of materials
structure? Macroscopic, mi… ?
4. What is the common characteristics of the atomic
structures of noble elements?
sp sub-shells are …?
A SIMPLE RULE FOR DETERMINING
ELECTRON CONFIGURATION
6s 6p 6d 6f
5s 5p 5d 5f
4s 4p 4d 4f
3s 3p 3d
2s 2p
1s
Example:
1s2 2s2 2p6 3s2 3p6 4s2 3…..
Special circumstances:
spn hybrid orbital, n= 1,2, or 3, that
causes an exception to this rule. P-22 see
foot note table 2.2 which elements
more prone to hybridization?
Reason: lower energy states.
Let’s do a class exercise: Look at the periodic table (inside back
cover) and select a material, say Chromium (Symbol= ?, Z= ?, A= )
Now start the electron configuration process using the rule/chart above:
1s2 2s2 ……………………Now look at Table 2.2 for Cr, p-25

3. • Why? Valence (outer) shell usually not filled completely.
• Most elements: Electron configuration not stable.
Element
Hydrogen
Helium
Lithium
Beryllium
Boron
Carbon
...
Neon
Sodium
Magnesium
Aluminum
...
Argon
...
Krypton
Atomic #
1
2
3
4
5
6
10
11
12
13
18
...
36
Electron configuration
1s1
1s2 (stable)
1s22s1
1s22s2
1s22s22p1
1s22s22p2
...
1s22s22p6 (stable)
1s22s22p63s1
1s22s22p63s2
1s22s22p63s23p1
...
1s22s22p63s23p6 (stable)
...
1s22s22p63s23p63d104s246 (stable)
SURVEY OF ELEMENTS (See p-25)
So ?
• Rows: Periods ---- Columns: Groups
Electropositive elements:
Readily give up electrons
to become + ions.
Electronegative elements:
Readily acquire electrons
to become - ions.
He
Ne
Ar
Kr
Xe
Rn
inertgases
accept1e
accept2e
giveup1e
giveup2e
giveup3e
FLi Be
Metal
Nonmetal
Intermediate
H
Na Cl
Br
I
At
O
SMg
Ca
Sr
Ba
Ra
K
Rb
Cs
Fr
Sc
Y
Se
Te
Po
THE PERIODIC TABLE
Analyze Periodic Table Characteristics
• Horizontal rows – Period – repeats structure/characteristics
• Vertical Columns = Groups: similar valence/bonding electron,
properties- changes gradually horizontally and vertically down
• Group 0- inert gas-
• VIIA (halogens), VIA – 1 , 2 electron short of complete shell
• IA, IIA – Alkali/Alkaline Earth Metals- 1, 2 electron excess in
orbit than complete shell –
• IIIB through IIB in 3 long periods – Transition metals.
d- subshells are partially filled, sometimes with 1,2
electrons in higher subshells
• IIIA, IVA, VA – intermediate between metal and nonmetal
because of their valence electron structure.
ELECTRONEGATIVITY
• Electronegativity values assigned: from 0.7 to 4.0
• Large values: more tendency to acquire electrons.
• Electropositive Elements- readily give up electrons
to become electropositive ions. E.g. metals
• Electronegativity: readiness to accept electrons to form
negatively charged ions.
See Figure in Text

4. 7
Smaller electronegativity Larger electronegativity
He
-
Ne
-
Ar
-
Kr
-
Xe
-
Rn
-
F
4.0
Cl
3.0
Br
2.8
I
2.5
At
2.2
Li
1.0
Na
0.9
K
0.8
Rb
0.8
Cs
0.7
Fr
0.7
H
2.1
Be
1.5
Mg
1.2
Ca
1.0
Sr
1.0
Ba
0.9
Ra
0.9
Ti
1.5
Cr
1.6
Fe
1.8
Ni
1.8
Zn
1.8
As
2.0
ELECTRONEGATIVITY (contd.)
• How about noble elements!
Elements are more likely to attract electrons if their outer shell
is almost full, and closer to the nucleus. Which means
diagonally from B-L to T-R.
Atomic Bonding
in Solids
Bonding- determines
many physical
properties such as
elasticity, strength,
melting temp. etc.
Interatomic separation
vs repulsive,
attractive and net
(a) forces and
(b) (b) energies.
At distance r0,
net force = ?
Net Energy = ?
Bonding Types
Bonding
Primary/chemical Secondary
Note: Both Primary and secondary bonding can and do exist simultaneously
Review
• How many quantum numbers ?
• Hybrid orbital spn - effect/reason ?
• Electronegativity - ? Willingness to accept ??
• When bonded, Net Force = ?
» Net Energy = ?
• Halogens in Group ?
• Alkali Metals in Group ?

5. Na (metal)
unstable
Cl (nonmetal)
unstable
electron
+ -
Coulombic
Attraction
Na (cation)
stable
Cl (anion)
stable
• Occurs between + and - ions. Non-directional
• Requires/powered by electron transfer.
• Larger difference in electronegativity required.
• Example: NaCl
IONIC BONDING
• High Melting Temperature Tm , hard, brittle, insulator
• Predominant bonding in Ceramics
Give up electrons Acquire electrons
He
-
Ne
-
Ar
-
Kr
-
Xe
-
Rn
-
F
4.0
Cl
3.0
Br
2.8
I
2.5
At
2.2
Li
1.0
Na
0.9
K
0.8
Rb
0.8
Cs
0.7
Fr
0.7
H
2.1
Be
1.5
Mg
1.2
Ca
1.0
Sr
1.0
Ba
0.9
Ra
0.9
Ti
1.5
Cr
1.6
Fe
1.8
Ni
1.8
Zn
1.8
As
2.0
CsCl
MgO
CaF2
NaCl
O
3.5
Adapted from Fig. 2.7, Callister 6e.
EXAMPLES: IONIC BONDING
• Requires shared electrons
• Example: Methane CH4
C: has 4 valence e,
needs 4 more
(to complete outer shell)
H: has 1 valence e,
needs 1 more
Bonding is DIRECTIONAL
• Electronegativities
are comparable.
shared electron
from carbon ato
shared electron
from hydrogen
atoms
H
H
H
H
C
CH4
COVALENT BONDING
• Most bonds are partially ionic partially covalent
%ionic bonding={1-exp[-(.25)(XA-XB)2]}x100
X-electronegativity of A or B elements. See Table 2.7 p-24
• Molecules of nonmetals
• Molecules with metals and nonmetals
• Elemental solid molecules (RHS of Periodic Table)
• Compound solids (about column IVA)
He
-
Ne
-
Ar
-
Kr
-
Xe
-
Rn
-
F
4.0
Cl
3.0
Br
2.8
I
2.5
At
2.2
Li
1.0
Na
0.9
K
0.8
Rb
0.8
Cs
0.7
Fr
0.7
H
2.1
Be
1.5
Mg
1.2
Ca
1.0
Sr
1.0
Ba
0.9
Ra
0.9
Ti
1.5
Cr
1.6
Fe
1.8
Ni
1.8
Zn
1.8
As
2.0
SiC
C(diamond)
H2O
C
2.5
H2
Cl2
F2
Si
1.8
Ga
1.6
GaAs
Ge
1.8
O
2.0
columnIVA
Sn
1.8
Pb
1.8
EXAMPLES: COVALENT BONDING
Usual
examples
Bonds diagonally farther = higher ionic bond, closer = higher covalent bond

6. • Arises from a sea of donated valence electrons
(1, 2, or 3 from each atom).
• The electron fluid/cloud acts as ‘glue’ for the ‘ion cores’
• Primary bond for metals
and their alloys
• Atoms stack orderly-
forms crystal structure.
• Floating electrons =
carriers of energy (heat,
electricity, )
+ + +
+ + +
+ + +
METALLIC BONDING
Electron cloud (glue)
Nucleus (ion core)
Arises from interaction between dipoles (2 charged poles)
• Fluctuating dipoles – temporary
• Weak bond
HH HH
H2 H2
secondary
bonding
ex: liquid H2asymmetric electron
clouds
+ - + -secondary
bonding
SECONDARY BONDING
Examples- liquids
Bonding between noble element molecules
SECONDARY BONDING (Contd.)
• Permanent dipoles-molecule induced
+ - secondary
bonding
+ -
H Cl H Clsecondary
bonding
secondary bonding
-general case:
-example: liquid HCl
-example: between polymer
chains
Polar
Molecules
SECONDARY BONDING (Contd.)
• Hydrogen-bond
-A special case of polar molecule bonding
-Strongest of secondary bonding
-Occurs between molecules where H is covalently bonded
to F (e.g. HF) ,O (e.g. H2O), or N (e.g. NH3).
-Directional ?
Water
O
HH

7. 14
Type
Ionic
Covalent
Metallic
Secondary
Bond Energy
Large!
Variable
large-Diamond
small-Bismuth
Variable
large-Tungsten
small-Mercury
smallest
Comments
Nondirectional (ceramics)
Directional
(semiconductors, ceramics
polymer chains)
Nondirectional (metals)
Directional
inter-chain (polymer)
inter-molecular
SUMMARY- BONDING
15
• Defn: Bond length, r
• Defn: Bond energy, Eo
F
F
r
• Melting Temperature, Tm
Eo=
“bond energy”
Energy (r)
ro
r
unstretched length
r
larger Tm
smaller Tm
Energy (r)
ro
Tm is larger if Eo is larger.
PROPERTIES FROM BONDING: TM
Melting Temperature of some Compounds
IonicDistance [Å] MeltingTemperature [°C]
NaF 2.31 988
NaCl 2.81 801 Na Halides
NaBr 2.98 755
NaI 3.23 651
MgO 2.1 2800
CaO 2.4 2580 Oxides
SrO 2.57 2430
BaO 2.76 1923
LiF 2.01 824
NaF 2.311 988 Florides
KF 2.67 846
RbF 2.82 775
16
• Elastic modulus, E
• E ~ curvature at ro
cross
sectional
area Ao
L
length, Lo
F
undeformed
deformed
LF
Ao
= E
Lo
Elastic modulus
r
larger Elastic Modulus
smaller Elastic Modulus
Energy
ro
unstretched length
E is larger if
Eo is larger.
PROPERTIES FROM BONDING: E

8. 16
• E ~ slope (blue line) of Net Force curve (red) at zero F
More vertical slope means stiffer (high E) material
More horizontal slope means more flexible material.
PROPERTIES FROM BONDING:
E(contd.)
Another way is to examine the bonding force curve.
17
• Coefficient of thermal expansion,
• ~ symmetry at ro
is larger if Eo is smaller.
L
length, Lo
unheated, T1
heated, T2
= (T2-T1)
L
Lo
coeff. thermal expansion
r
smaller
larger
Energy
ro
PROPERTIES FROM BONDING:
18
Ceramics
(Ionic & covalent bonding):
Metals
(Metallic bonding):
Polymers
(Covalent & Secondary):
secondary bonding
Large bond energy
large Tm
large E
small
Variable bond energy
moderate Tm
moderate E
moderate
Directional Properties
Secondary bonding dominates
SUMMARY: Properties
Tm
E
Guess
these 3
properties
small
small
large